Philonotis uncinata is an
extremely variable species characterized by its dioicous condition, gemmiform perigonia,
papillae projecting from the upper ends of the leaf cells, and a costa that can
be subpercurrent, percurrent or shortly excurrent. It is a small to
medium-sized species, with short, lax leaf cells and basal cells near the costa
commonly somewhat enlarged. It has recurved leaf margins that are usually
doubly serrate by either blunt or sharp teeth. The species is extremely
variable in terms of costal length, leaf apex shape and leaf areolation. There
are three distinctive morphologies commonly associated with P. uncinata
and unless the complex is studied in detail (see Florschütz 1964, Zales 1973,
Griffin 1994) it is difficult to accept them as representing a single species.
Plants with more or less obtuse leaves, subpercurrent costae, blunt marginal
leaf serrations, and large, lax-walled leaf cells represent the gracillima-expression.
Plants with acute leaves, percurrent costae, blunt or sharp marginal leaf
serrations and lax-walled leaf cells represent the glaucescens-expression.
Plants with acute leaves, short-excurrent costae, sharp marginal leaf
serrations and firm-walled leaf cells represent the uncinata-expression.
Philonotis uncinata keys out near P. elongata, but it is
distinguished from that species by its smaller size, blunter marginal leaf
serrations, and short, erect, unbranched stems. Philonotis uncinata
is more easily confused with P. hastata through its gracillima-expression
or P. sphaericarpa through its uncinata-expression. The gracillima-expression
of P. uncinata differs from P. hastata in having narrower,
firmer-walled leaf cells and more strongly papillose leaf cells, while the uncinata-expression
of P. uncinata differs from P. sphaericarpa in its smaller sized
and in having a smooth, excurrent costa rather than a toothed one.

Although it is not yet known from
Central America, P. marchica(Hedw.) Brid. has been found in Mexico. This
species resembles the uncinata-expression of P. uncinata, it
differs from it in having mostly plane leaf margins with singly serrate teeth.